As shown in FIG. 1, generally, in a method of forming a strip using a twin-roll strip caster, molten steel stored in a ladle 1 is introduced into a tundish 2, and is then fed to a space between edge dams 5 provided at both ends of casting rolls 6, that is, a space between the casting rolls 6, through an injection nozzle 3 to start solidification. In this case, a meniscus shield 4 is installed over the casting rolls 6 to protect the molten steel pool and prevent the oxidation of the molten steel. Subsequently, the molten steel is formed into a strip 8 while passing through a roll gap 7 between the casting rolls 6, and then the strip 8 is drawn, cooled, and then wound by a winder 9.
As such, in the method of forming molten steel into a strip having a thickness of 10 mm or less using a twin-roll strip caster, it is important that molten steel rapidly passes between the water-cooled casting rolls 6 rotating in directions opposite to each other through the injection nozzle 3, and thus a non-cracked strip having a desired thickness may be produced in high yield.
Meanwhile, the surface of a continuous casting mold is treated using an electrolytic plating method. As shown in FIG. 2, a casting roll 6, which is a subject to plating, is connected to a cathode, and a metal 10, which is used to plate the subject, is connected to an anode. Thereafter, the subject is completely immersed into a plating solution containing the plating metal, and then electric current is applied to this system, thereby obtaining a plating layer having a desired thickness.
In this case, the casting roll 6, which is the subject to plating, is rotated in order to improve plating quality, and, if it is flat, a plating solution is rotated to form a uniform plating layer. In this case, primarily, a nickel plating layer is formed on a copper plate, and secondarily, a nickel-tungsten (Ni—W) plating layer, a nickel-cobalt (Ni—Co) plating layer, which is a high-hardness plating layer, or the like is formed thereon to improve durability.
In this conventional method of plating a mold, nickel (Ni) is chiefly used, and, in relation to plating conditions, such as current density, temperature of a plating solution, and the like, the following technologies are disclosed.
Japanese Unexamined Patent Application Publication No. 1998-066049 discloses a technology for improving the durability of a casting roll by spray-coating the end surface of the casting roll. Japanese Unexamined Patent Application Publication No. 1989-254357 discloses a technology for improving the durability and quality of a mold by primarily forming a nickel (Ni) plating layer on the surface of the mold and then secondarily forming a chromium (Cr) plating layer on the nickel (Ni) plating layer. Japanese Unexamined Patent Application Publication No. 1990-047890 discloses a technology for improving the quality of a mold by forming a nickel (Ni) plating layer on the surface of the mold and then decreasing a heat transfer coefficient through a graphite coating process and thus decreasing temperature variability. Japanese Unexamined Patent Application Publication No. 2001-205399 discloses a technology for improving the durability of a casting roll by primarily forming a nickel (Ni) plating layer on the casting roll and then secondarily forming a nickel-tungsten (Ni—W) plating layer, a nickel-cobalt (Ni—Co) plating layer or the like, which is a high-hardness plating layer, on the nickel (Ni) plating layer.
However, in these conventional technologies, basically, a nickel (Ni) plating layer is primarily formed, and then is secondarily hard-plated or spray-coated for protection. In particular, they are problematic in that since a hard plating layer has high hardness, but on the other hand, it has high internal stress, it is very sensitive to be cracked, and as a result it is limited in so far as its thickness must be thin, with the result that it cannot be easily applied to extremely worn portions of the subjects to be plated.
Therefore, due to the limitation of the thickness of the hard plating layer, there are problems in that the hard layer cannot be easily applied to the end surface of a casting roll, the end surface thereof being worn out by being brought into contact with the edge dam, and in that the durability of the casting roll is deteriorated.
Here, among reference numerals, which are not described, in FIG. 2, ‘20’ is a plating bath, and ‘40’ is a rectifier.